The present invention relates to an agent for inhibiting elution of phosphorus, which is dumped into sludge in lakes, rivers, canals or sea to insolubilize and fix phosphorus that exist in sludge and elutes under reducing conditions, a method of manufacturing the same, and a method of inhibiting elution of phosphorus in sludge by use of the same.
In sludge in lakes, rivers, canals and sea, there exist phosphorus compounds originating mainly from household sewage. Though depending on places, their amounts are relatively large. This type of phosphorus compounds normally exist as a solid. But if the environment of the sludge turns to reducing conditions, they elute into water. When organic matters in sludge decompose, oxygen is consumed, so that reducing conditions often reveal. Phosphorus that has eluted into water causes water blooms or red tides. Thus it has to be removed.
Heretofore, in order to eliminate influence of phosphorus compounds in water, removing sludge has been considered to be the only way and dredging has been carried out. But dredging not only gives damage to benthos and waterweeds, thus damaging the biosphere. No expected results can be achieved by dredging because sludge tends to scatter into water. Further, there is a problem of how the sludge taken out by dredging should be disposed of.
Therefore, efforts are being done to fix phosphorus compounds in sludge so that they will not elute even under reducing conditions and thus will not cause eutrophication. For example, in Shiga prefecture, in order to improve the water quality of Lake Biwa and to prevent generation of water blooms and freshwater red tides, trials are being made to inject an aqueous solution of calcium nitrate into sludge. According to an article on the front page of the Mainichi Shimbun newspaper, Oct. 5, 1997 edition, as substances for which one can expect functions as oxidants, ferric chloride, alum and calcium nitrate were selected, their aqueous solutions were added into sludge, and it was examined to what extent phosphorus in sludge eluted. The following results were obtained.
These data show that by injecting a 0.7% aqueous solution of calcium nitrate, elution of phosphorus can be inhibited by as much as 98%.
But injection of such an aqueous solution into sludge is low in effect unless it is suitably carried out, so that it is necessary to manufacture a special device suitable for such work. The abovesaid newspaper article carries photos of a device having numerous nozzles planted in a steel plate. If they are used in the form of a liquid, due to dispersion into water during injection, calcium nitrate will not necessarily function with high efficiency. Further, while the effect of calcium nitrate can be expected after injection, it is poor in sustainability, so that injection has to be frequently carried out.
An object of the present invention is to provide an agent for inhibiting elution of phosphorus which can be used not by injection of an aqueous solution but in an easier way, is low-cost and high in the effect and sustains its effect for a longer period of time. Another object of the present invention is to provide a method of manufacturing such an agent for inhibiting elution of phosphorus, and a method of inhibiting elution of phosphorus present in sludge.
The agent for inhibiting elution of phosphorus of the present invention comprises as its major component an oxidant and a soluble calcium salt, and a hydrophobic substance that delays elution of the soluble calcium salt, and a binder, the composition being formed into tablets or briquette. It inhibits elution of phosphorus in sludge in rivers, lakes and sea by fixing it.
The method of manufacturing the agent for inhibiting elution of phosphorus according to the present invention comprises the steps of mixing a composition which comprises an oxidant and a soluble calcium salt as major components, a hydrophobic substance for delaying elution of the soluble calcium salt and a binder, and if necessary, contains ferric chloride, putting the mixture in a mold, and pressing it into tablets or briquette.
The method of the present invention for inhibiting elution of phosphorus present in sludge using the agent for inhibiting elution of phosphorus comprises putting the tablets or briquette of the agent for inhibiting elution of phosphorus into sludge in lakes, rivers, canals or sea to fix the phosphorus.